Platelet-Endothelial Cell Adhesion Molecule-1 (PECAM-1, CD31) is an adhesion and signaling molecule that is expressed on endothelial cells (EC), leukocytes, and platelets. It is a key molecule in regulating transendothelial migration of leukocytes, however little in known about the mechanisms involved or its role in lung disease, despite high levels of expression in the pulmonary circulation. Using PECAM-1 knockout (KO) mice, the investigators have found that lung neutrophil infiltration in immune complex deposition disease is highly PECAM-dependent. The goals of this grant are to define the mechanisms by which PECAM regulates transmigration using in vivo and in vitro models and to define the role of PECAM-1 in other models of lung disease. The hypotheses to be tested are that PECAM regulates transmigration through endothelial and neutrophil signaling events mediated by localized regions of the PECAM cytoplasmic domain and that PECAM-1 will play an important role in only specific types of lung injury. Three independent, but related specific aims are proposed: In Aim 1, the mechanisms by which PECAM-1 regulates migration of neutrophils into the lung will be determined using bone marrow chimeras generated between wild type and KO animals and by reconstituting PECAM KO mice with specific PECAM-1 isoforms that have mutations in their cytoplasmic domains. In Aim 2, the mechanisms by which PECAM-1 regulates transendothelial migration will be defined using in vitro systems that include a human lung EC transmigration model and experimental transmigration models in which mutant forms of PECAM-1 will be transfected into a PECAM-negative human endothelial-like cell line or into PECAM-negative murine EC's derived from the KO mice. In Aim 3, anti-PECAM-1 antibodies and PECAM knockout mice will be used to test the importance of PECAM-1 in regulating leukocyte transmigration into the lung in "endothelial-driven" vs. "leukocyte-driven" lung inflammation, in models of oxidant-induced lung injury, and in other immune-mediated lung injury models. These studies will fill a gap in the basic understanding of lung and cell adhesion biology, as well as provide potentially useful therapeutic information.